1972 MG MGB Problems: 2 Known Issues

On the 1962-1980 MG MGB 1800 B-series I4, the MGB is monocoque (unibody), so the rocker/sill assembly is a stressed structural member, not just trim. The factory built the sill as three layers — outer sill skin, inner sill, and the boxed 'castle rail' truss between them — with poor sealing at the seams. Road dirt packs into the sill, the inner cavity is never drained or waxed, and moisture sits trapped against bare steel. The worst pocket is where the rear of the sill meets the rear wheel-arch — an almost inch-deep open trap pointing down-and-back. Once the castle rail and inner sill rot, the shell loses rigidity: the doors drop and stop latching, the door gaps go uneven, and the whole body flexes and 'scuttle shakes' over bumps. Other classic rust traps feed the same problem: the front wing dogleg ahead of the windscreen, the inner-wing valance ledge, the headlamp bowls and the floor edges.

On the 1968-1980 MG MGB 1800 B-series I4, the MGB's cooling system has little reserve margin even when healthy, and decades of scale silt up the radiator core and clog passages in the block, dropping flow and capacity. Later cars are worse: emissions-era retarded timing, the larger/heavier-breathing engine, and (on US cars) the deletion of the earlier engine-driven fan in some setups all add heat. The result is the well-known tendency to creep hot in summer traffic, on hills, or at idle — risking boil-over, head-gasket failure, and warped heads. A blocked or under-capacity radiator is usually the real culprit, not the thermostat.